An optically active naphthalene compound of the following formula [I]:
or a pharmaceutically acceptable salt thereof or a hydrate thereof shows an excellent selective phosphodiesterase 4 (PDE4) inhibitory activity, thereby is useful as an agent for treatment of an inflammatory skin disease such as atopic dermatitis and the like (Patent Literature 1). A below-mentioned method for preparing said compound in an industrial scale is proposed in Patent Literature 2.

In the above scheme, Z represents a hydroxy-protecting group such as tert-butyldimethylsilyl group.
While the optically active tetrahydroquinoline compound (b) is an essential substrate compound in the above-mentioned method, the Patent Literature 2 discloses a synthetic process for producing said compound comprising the below-mentioned four steps.

In the above scheme, Za represents benzyloxycarbonyl group and Z represents a hydroxy-protecting group such as tert-butyldimethylsilyl group.
The method disclosed in Patent Literature 2 is therefore that for producing the optically active naphthalene compound [I] which consists of seven steps including four steps necessary for producing the optically active tetrahydroquinoline compound (b).